1. Field of the Invention
The invention relates to a method of producing a silver halide photographic emulsion, an apparatus for the same, and also a method of measuring a silver or halogen ion concentration of an emulsion during or after the formation of a silver halide photographic emulsion, and an apparatus for the same.
2. Description of the Related Art
As a prior art technique, U.S. patent specification No. 3,031,304 discloses a method of producing a fine particle emulsion which has a mean particle diameter of 0.06 xcexcmm. In the specification, a convenient method is disclosed in which particles are formed in a pAg range of about 2 to 6 by using a method of simultaneously mixing a silver nitrate solution and a potassium bromide solution which are reaction liquids, and four pumps are used for injecting the reaction liquids so as to automatically control the pAg. Specifically, the silver nitrate solution and the potassium bromide solution are separately provided with a pump so as to be injected in a substantially stoichiometrically equivalent amount. A potentiometer circuit has a limit switch which, when a silver ion concentration of an emulsion in a precipitation solution is raised to pAg of 5 or more, functions so as to decrease the amount of potassium bromide pumped to be injected, by 1% by means of a third pump. When pAg reaches to 5 or more, the third pump is stopped. When pAg is lowered to a predetermined level, usually 4.3 or less, potassium bromide to be injected is added by the third pump.
The fourth potassium bromide injecting pump is used for a manual addition. In accordance with the reading of the potentiometer or a recorder, the operator can adequately adjust the addition of potassium bromide.
Furthermore, U.S. patent specification No. 3,821,002 discloses a control apparatus and a method of producing a silver halide emulsion. In the apparatus and method, pAg in a precipitation bath is made constant or changed, and the flow rates of a silver nitrate solution and a halogen sale solution to be added are changed in accordance with a program so that the required accuracy of pAg is maintained.
Furthermore, Photogr. Korresp. 101, 37 (1965) teaches relationships of crystal diameters of silver halide and the number of particles which are obtained by maintaining the temperature, and adjusting valves for adding a silver nitrate solution and a halogen salt solution, by an electrical control, thereby controlling pAg and pH.
All the three publications relate to a control of a preset target value, and teach only that several minutes must be elapsed in the period from the uncontrolled state at the start of reaction to the control of the pAg value stabilized at the preset target value, and the controlled state is unstable. The publications say nothing about a control method on apparatus-for causing the pAg value to rapidly reach a preset target value and conducting a control at the preset target value.
Japanese Patent Unexamined Publication No. SHO 61-65305 discloses an optimum control method in which a defect of the conventional PID control is eliminated and a computer control is done in accordance with a mathematical model. Japanese Patent Unexamined Publication No. HEI 5-181504 discloses an adaptive control method having a feedforward element in which a sequential plant model in the control of a physical quantity of a system is estimated, the control is conducted on the basis of the plant model, a variation quantity at an elapse of a dead time with respect to a variation externally applied to the system is predicted by using a variation pattern of a physical quantity which causes the external variation, and the external variation at an elapse of a dead time is previously canceled. U.S. patent specification No. 4,933,870 discloses a method of producing a silver halide emulsion which employs an apparatus and method of converting an output signal of a nonlinear ion sensor into a linear signal. U.S. patent specification No. 5,248,577 discloses an apparatus and method of producing a silver halide emulsion in which the density of halogen ions and flow rates of added halogen salt and silver nitride solutions are periodically measured, the measured data are accumulated, an internal calculation is conducted by an equation estimated on the basis of the accumulated data, and the flow rates of the added halogen salt and silver nitride solutions are controlled. In these disclosed techniques, complex calculations are done, and hence it is difficult for a controller or computer which is commercially available, to conduct processing with a short period. These publications make no mention of a method or apparatus which is used for starting the control from the uncontrolled state. When the pAg distribution in a precipitation vessel is uniform, a conventional PID control can sufficiently cope with the control at a steady state far as disturbance is not extremely produced.
However, relationships between a potential EAg corresponding to the silver ion activity and ion concentrations of silver nitride and halogen salt (e.g., potassium bromide) in a liquid containing silver halide crystals are linear and abruptly changed at the equivalence point as shown in FIG. 1. In the EAg range of xe2x88x9250 mV to +150 mV where precipitation of a silver halide emulsion is often conducted while controlling the silver ion concentration, a very small change in concentration of silver ions or halogen ions causes the potential to be abruptly changed. Even when, in the uncontrolled state at the start of precipitation, the control is to be conducted at the preset target EAg potential, pAg in a conventional precipitation bath is largely changed in an initial period of precipitation and hardly converged into the target value, with the result that several minutes must be elapsed before pAg is stabilized. Furthermore, the potential locus of EAg obtained until the controlled state is attained cannot be reproduced.
As the scale of a precipitation bath is increased, the control is further unstabilized, and hence it is difficult to stably produce a silver halide emulsion of constant quality.
Also, conventionally, in order to obtain desired photographic characteristics, it is essential to control the silver or halogen ion concentration during or after the formation of a silver halide photographic emulsion, and a technique is widely employed in which reference and indicator electrodes for the above-mentioned control are directly inserted into a precipitation vessel in which a halogen salt aqueous solution reacts with a silver nitrate aqueous solution and which contains a gelatin aqueous solution.
The relationships between a silver or halogen ion concentration and an electrode potential is described in xe2x80x9cThe Theory of the Photographic Process, Third edition or Fourth edition (Macmillan Publishing Co., Inc.)xe2x80x9d.
Silver and halogen ion concentrations are respectively defined by equations (1) and (2):
pAg=xe2x88x92log[Ag+]xe2x80x83xe2x80x83(1)
xe2x80x83pX=xe2x88x92log[Xxe2x88x92]xe2x80x83xe2x80x83(2)
where [Ag+] indicates the silver ion activity, and [Xxe2x88x92] indicates the halogen ion (Brxe2x88x92, Clxe2x88x92, or Ixe2x88x92) activity.
The electrode potentials EAg and EX in relation to the silver or halogen ion activity in silver halide crystals are expressed as follows:
EAg=Exc2x0Agxe2x88x922.30259xc3x97(RT/F)xc3x97pAgxe2x80x83xe2x80x83(3)
where Exc2x0Ag indicates the standard potential for a silver half cell, R indicates the gas constant, F indicates the Faraday constant, and T indicates an absolute temperature.
In a silver halide emulsion which is practically used, halogen halide is often in excess, and hence a silver indicator electrode is covered by a silver halide layer and saturated with silver halide salt. Therefore, the silver ion and halogen ion activities on the surface of the electrode have the relationship of equation (4) below,
[Ag+][Xxe2x88x92]=Kspxe2x80x83xe2x80x83(4)
where Ksp indicates the solubility product of silver halide.
In other words, a silver/silver halide electrode is essentially equivalent to a silver electrode in which the silver ion activity is governed by the halogen ion activity in a solution.
Therefore, Ex is expressed by equation (5) below, but an indicator electrode in a silver halide emulsion indicates the same potential because the emulsion solution is in equilibrium with silver halide crystals.
xe2x80x83Ex=Exc2x0AgX+2.30259xc3x97(RT/F)xc3x97pXxe2x80x83xe2x80x83(5)
EAg=Ex (it is assumed that Ex=E)xe2x80x83xe2x80x83(6)
The electrode potential E can be measured by forming a cell system in combination with a potential ER of a reference electrode which produces the reference potential, and detecting a potential difference. The relationships between E and pAg and PX can be expressed by the following equations:
E=Exc2x0Agxe2x88x92ERxe2x88x922.30259xc3x97(RT/F)xc3x97pAgxe2x80x83xe2x80x83(7)
E=Exc2x0AgXxe2x88x92ERxe2x88x922.30259xc3x97(RT/F)xc3x97pXxe2x80x83xe2x80x83(8)
Therefore, the states of pAg and PX of a silver halide photographic emulsion can be grasped by measuring the potential E of the indicator electrode.
When the reference electrode which functions as the reference of a potential measurement is inserted into a measured liquid, however, the temperature variation of the measured liquid causes a long period to be elapsed before a constant potential is obtained. Therefore, it is impossible to continuously measure instantaneous variations of an ion concentration, gelatine and silver halide particles adhere to the liquid junction of the reference electrode to clog the liquid junction, whereby an asymmetry potential is produced so that it is difficult to obtain a constant potential which functions as the reference. (See xe2x80x9cPhotographic Emulsion Chemistry 1966xe2x80x9d by G. F. Duffin, p. 14, FOCAL PRESS LIMITED.)
When a silver ion activity of a system such as a gelatin aqueous solution containing the silver halide crystals is measured with using a conventional silver metal rod as an indicator electrode, the reproducibility of the measured potential in repeated measurements is not always satisfactory. Furthermore, silver halide crystals obtained from the system vary in size distribution, shape, photographic characteristics, etc.
Japanese Patent Unexamined Publication No. SHO 60-213858 discloses a method in which, as a countermeasure for stabilizing a conventional electrode for detecting a silver ion concentration in order to obtain stabilization of the indicator electrode, an alloy electrode made of silver and a metal of one or more kind which is nobler than silver, or of metals of two or more kinds which are nobler than silver is used.
In the method disclosed in Japanese Patent Examined Publication No. SHO 60-213858, however, a measured liquid penetrates into a small gap between the metal silver and its alloy which function as the indicator electrode, and a holding cover for the electrode, and adheres to the electrode and the cover. When liquids of different kinds are to be measured, therefore, it is impossible to obtain an accurate value.
The present invention has been made to solve the above-mentioned problems, and therefore an object of the invention is to provide a method and apparatus in which, in formation of a silver halide photographic emulsion while controlling the silver ion concentration in precipitation of a silver halide emulsion in a precipitation bath, the system can rapidly attain a potential corresponding to the silver ion activity with excellent reproducibility after starting from an uncontrolled state at the start of precipitation, and a silver halide photographic emulsion can be produced while conducting a control at the preset target value.
Another object of the invention is to provide a method and apparatus for measuring a silver or halogen ion concentration in which the temperature variation of a reference electrode is eliminated so as to ensure a constant reference potential, and the manner of mounting an indicator electrode is improved so that the measurement is always correctly performed, whereby the silver or halogen ion concentration during the formation of a silver halide photographic emulsion can be measured instantaneously with excellent reproducibility and the reaction state of the formation of silver halide crystals can be traced correctly.
In order to solve the above-mentioned objects, a first aspect of the invention has been achieved by the provision of a method of producing a silver halide photographic emulsion in which a silver ion concentration in precipitation of a silver halide emulsion in a precipitation bath is controlled, wherein a precipitation bath in which stirring is conducted rapidly and uniformly, and crystal formation and crystal growth are uniformly performed is used, and the method comprises the steps of: in a start period of precipitation, quantitavely adding a silver nitrate solution and a halogen salt solution at a constant ratio flow rate; when an EAg value reaches a designated EAg value region in the vicinity of a preset target EAg value, starting a control of an adding rate of the halogen salt solution by using a controller which has an operation period equal to or shorter than 1 sec.; and, after holding a tuning parameter of a proportional, integral and differential (PID) action controller to a minimum response level, conducting a control in which the tuning parameter is switched to an optimum control tuning parameter which corresponds to the preset target value and a solute rate of silver/halogen ions to be added.
In the above-mentioned method of producing a silver halide photographic emulsion, the tuning parameter is previously estimated in accordance with a simulation based on a plant model, whereby a calculation period of a control system is eliminated and a control response speed is increased, and a control using a direct digital loop controller (DDLC) is conducted.
In the above-mentioned method of producing a silver halide photographic emulsion, the precipitation bath in which stirring is conducted rapidly and uniformly and crystal formation and crystal growth are uniformly performed is a precipitation bath in which the silver nitrate solution and the halogen salt solution are separately supplied through a lower end portion of a mixing chamber consisting of a casing, the solutions are diluted with a colloid aqueous solution charged in the mixing chamber, both the reaction solutions are abruptly stirred by first stirring means to react with each other, thereby forming silver halide particles, the silver halide particles are immediately or within 1 sec. or shorter to be discharged into a colloid aqueous solution existing outside and above the mixing chamber and in the precipitation bath, and the silver halide particles are aged.
Also, the first aspect of the invention has been achieved by the provision of an apparatus for producing a silver halide photographic emulsion in which a silver ion concentration in precipitation of a silver halide emulsion in a precipitation vessel is controlled, wherein the precipitation bath comprises: a silver nitrate solution tank and a halogen salt solution tank which are separately disposed outside the precipitation bath; a mixing chamber consisting of a casing, the casing being disposed at a position where is in the center of the precipitation bath which is filled with a colloid aqueous solution, and close to a bottom of the bath, an interior of the casing being filled with the colloid aqueous solution, upper and lower ends of the casing being opened, an impeller being disposed inside the casing, supply ports for a silver nitrate solution and a halogen salt solution being disposed in a lower end portion of the mixing chamber; first stirring means for rapidly mixing both the reaction solutions and causing the solutions to react with each other, the stirring means being disposed in a lower interior portion of the mixing chamber; and a second stirring means for immediately upward discharging formed silver halide particles to an outside of the mixing chamber, the stirring means being disposed in an upper interior portion of the mixing chamber, an adding system for the silver nitrate solution is provided with constant flow rate holding means, three flow rate controlling means are connected in parallel to an adding system for the halogen salt solution which corresponds to the adding system for the silver nitrate solution, two of the flow rate controlling means are respectively provided with constant flow rate holding means, the other flow rate controlling means is provided with flow rate controlling means which is based on the electrode for detecting a silver ion activity, the other constant flow rate holding means comprises flow rate controlling means which is based on the electrode system, and the apparatus comprises a device which switches one of the constant flow rate holding means to the flow rate holding means based on the electrode system, in accordance with a preset potential corresponding to a designated silver ion activity.
In the invention, in order to use a precipitation bath in which stirring is conducted rapidly and uniformly and crystal formation and crystal growth are uniformly performed, the interior of the precipitation bath must be uniform so that the ion detection is correctly rapidly conducted, as a precondition for the method of producing a silver halide emulsion. To comply with this, for example, the method and apparatus disclosed in Japanese Patent Examined Publication No. SHO 55-10545 may be employed.
In the invention, the step of adding a silver nitrate solution and a halogen salt solution at a constant ratio or a constant flow rate means a process in which the solutions are quantitavely added by, for example, using a constant valve opening or an orifice plate functioning as the constant flow rate holding means.
In the invention, as means for controlling a silver nitrate solution and a halogen salt solution at a constant ratio or a constant flow rate, means for controlling the opening of a valve may be used, or, when a pump is used, means for controlling the number of revolutions of a motor for driving the pump may be used. In place of using a control of a control valve in the valve opening range where the flow rate can be adjusted most easily, therefore, the flow rate control may be conducted in accordance with a detected EAg value by controlling the number of revolutions of the motor for driving the pump.
In the invention, the designated EAg value region in the vicinity of a preset target EAg value means a region where the potential has a value of xc2x15 to xc2x160 mV, preferably xc2x110 to xc2x130 mV with respect to the potential EAg corresponding to the preset target pAg.
In the invention, the use of a controller which has a calculation period equal to or shorter than 1 sec. means an execution of a high speed control at 0.1 to 1.0 sec., preferably 0.2 sec. or shorter by using a direct digital loop controller of one loop, such as TOSDIC-211 manufactured by Toshiba Corporation, or YW-SERIES80 manufactured by Yokogawa Electric Corporation.
In the invention, the holding of a tuning parameter to a minimum response level means that the holding time when the proportional band is set to be 99.9% or more, the integral time is set to be 500 sec. or longer, and the derivative time is set to be from 0 sec. is set to be from 0.1 to 5 sec., preferably 0.5 to 1.0 sec.
In the invention, also when a pump is used, a pump for always controlling the flow rate, and that for adding a fixed amount of the major portion of the addition amount may be used independently from each other so that the addition is conducted by the sequence operation shown in FIG. 3.
A flow rate control valve may be driven by air, or alternatively means for adjusting the opening by a servomotor may be used.
Also, in order to solve the above objects, a second aspect of the invention has been achieved by the provision of a method of measuring a silver or halogen ion concentration wherein, in a sensor system which detects as a potential a silver or halogen ion concentration in a gelatin aqueous solution containing silver halide crystals, a reference electrode which functions as a reference of a potential measurement is inserted into a heat insulating bath without being directly inserted into the measured liquid, the bath being accurately controlled to have a constant temperature and electrically insulated, the measured liquid and the reference electrode are electrically connected with each other by a salt bridge, only one end portion of an indicator electrode is immersed into the measured liquid, the reference electrode and another end portion of the indicator electrode are connected with a potentiometer, and a potential is measured.
Also, the second aspect of the invention has been achieved by the provision of an apparatus for measuring a silver or halogen ion concentration, comprising: a reference electrode which is disposed in a heat insulating bath which has a constant temperature and is electrically insulated, only an end portion of the reference electrode being electrically connected with a gelatin aqueous solution containing silver halide crystals, by a salt bridge; an indicator electrode, only one end portion of the indicator electrode being immersed into the gelatin aqueous solution containing silver halide crystals; and a potentiometer which is electrically connected with the reference electrode and another end portion of the indicator electrode via a silver wire.
In the above-mentioned apparatus, a ceramic having micropores is used in a portion of the salt bridge, the portion making contact with the gelatin aqueous solution containing silver halide crystals, and a potassium nitrate solution is used as an inner liquid of the salt bridge.
Further, the second aspect of the invention has been achieved by the provision of an apparatus for measuring a silver or halogen ion concentration, wherein a silver metal rod of a purity of 99.9% or higher is used as the indicator electrode, platinum plating or an insulating material coating is applied onto a portion of the indicator electrode, the portion making contact with a holder unit, and the surface of the portion making contact with the gelatin aqueous solution containing silver halide crystals is plated by AgBr or Ag2S in a thickness of 0.1 xcexcm or less.
In the invention, the salt bridge between the gelatin aqueous solution containing silver halide crystals (hereinafter, referred to as xe2x80x9cmeasured liquidxe2x80x9d) and the reference electrode is preferably constructed of flexible plastic hose and a KNO3 solution is used as an inner liquid in the hose, and the concentration of the solution is from 0.01 to 5 Mol/l, preferably from 0.8 to 1.2 Mol/l.
In the invention, the immersion of only one end portion of the indicator electrode into the measured liquid means that only the tip end of one end portion of the indicator electrode is immersed and the body portion of the silver rod is not immersed into the measured liquid. The measurement of the potential is performed by measuring the potential difference between the reference electrode and the indicator electrode by means of a potentiometer.
In the invention, the heat insulating bath which has a constant temperature and is electrically insulated is preferably made of vinyl chloride or acrylic resin, such an chloroethylene or acrylic material, or provided with an insulation property and the inner liquid (the same as the salt bridge of inner liquid) of the vessel having an insulation property of 100 Mxcexa9 or higher maintained within xc2x10.5xc2x0 C. by a thermostatic chamber or the like, whereby the stability of the reference potential depending on the temperature is maintained.
In the invention, the use of ceramic having micropores in a portion of the salt bridge which makes contact with the gelatin aqueous solution containing silver halide crystals means that one end of the salt bridge is blocked by porous ceramic having a porosity from 2 to 40%, preferably from 5 to 15% so that the potassium nitrate solution which is the inner liquid passes through the ceramic plug to flow out from the heat insulating bath into the gelatin aqueous solution containing silver halide crystals in a flow rate from 0.01 to preferably 0.1 to 1 cc/min. at a head of 9.8 KPa. 10 cc/min., preferably from 0.1 to 1 cc/min. at a head pressure of 9.8 KPa.
In the invention, a silver metal rod of a purity of 99.9% or higher is used as the indicator electrode, and platinum plating or an insulating material coating is applied onto a portion of the silver metal rod which makes contact with the holder unit. As the insulating material, Teflon or ceramic is used. The silver metal rod is inserted through the holder unit and supported thereby via, for example, an O-ring. The surface of the one end portion which makes contact with the gelatin aqueous solution containing silver halide crystals is plated by AgBr or Ag2S in a thickness of 0.1 xcexcm or less. This allows the accuracy of the potential of the indicator electrode to be maintained.
Alternatively, a glass electrode may be used as the indicator electrode. In the alternative, a sensor system which can measure pH stably and with excellent reproducibility can be realized.